Bicycle crank arm parts/assembly and assembly tools

ABSTRACT

A crank arm for fitting to a crank fitting to the crank axle includes a crank axle mounting section having a centering structure for centering the crank axle mounting section to an end of a crank axle, and a rotary linkage member for nonrotatably fitting the crank axle mounting section to the end of the crank axle.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 08/890,744, filedJul. 11, 1997, now U.S. Pat. No. 6,276,885 which is a division ofapplication Ser. No. 08/687,203, filed Jul. 25, 1996, now U.S. Pat. No.5,845,543. This application also is closely related to application Ser.No. 08/891,698, now U.S. Pat. No. 5,819,599.

BACKGROUND OF THE INVENTION

The present invention is directed to a bicycle crank axle, a crank arm,and a crank set that offer high mounting precision and fasteningstrength, and that contribute to lighter weight. It also relates to abolt and assembly tools used for assembling these components.

In conventional mounting mechanisms for the crank arms and crank axle ina bicycle, both ends of the crank axle are shaped into essentiallysquare columns, a square hole is made in each crank arm, and the two arefitted together to fasten the crank arm onto the axle. The precision ofcentering afforded by the conventional mechanism for fitting togetherthe crank arms and the crank axle is inadequate, and the strength of thelinkage in the direction of rotation is inadequate as well. The problemof inadequate linkage strength is particularly notable where an aluminumalloy hollow pipe structure has been adopted for the crank axle, orwhere an aluminum alloy hollow structure has been adopted for the crankarms in order to reduce weight.

SUMMARY OF THE INVENTION

The present invention is directed to a bicycle crank axle, a crank arm,and a crank set that offer high mounting precision and fasteningstrength, but also contribute to lighter weight. Novel bolt and assemblytools used are for installing and removing these components so thatassembly and disassembly may be accomplished very easily.

In one embodiment of the present invention, a crank axle includes anaxle body having first and second ends, a first rotary linkage memberdisposed at the first end of the axle body for nonrotatably fitting afirst crank arm thereto, and a first centering structure disposed at thefirst end of the axle body in close proximity to the first rotarylinkage member. If desired, the rotary linkage member may be formed as aplurality of splines, and the same structure may be formed at the secondend. A crank arm for fitting to the crank axle includes a crank axlemounting section having a centering structure for centering the crankaxle mounting section to an end of a crank axle, and a rotary linkagemember for nonrotatably fitting the crank axle mounting section to theend of the crank axle. If desired, the rotary linkage member for thecrank arm may comprise splines that mesh with splines on the end of thecrank axle. Such a structure provides secure linking of the crank arm tothe crank axle without adding excessive weight.

To facilitate removal of the crank arm, a special bolt is used to fixthe crank arm to the crank axle. The bolt includes a head, a threadedsection having a diameter greater than or equal to the head, and aflange disposed between the head and the threaded section. The flangepreferably has a diameter greater than the diameter of the threadedsection, and the head defines a multiple-sided tool-engaging hole whichextends axially into the threaded section to maximize the tool engagingsurface. The crank axle mounting section of the crank arm preferablyincludes a stop ring or similar structure located so that the flange ofthe bolt is disposed between the stop ring and the first end of the axlebody. As a result, the bolt flange presses the crank arm apart from thecrank axle when the bolt is loosened.

Special tools may be provided to allow easy installation and removal ofthe components. For example, a tool that can be used for both fixing theaxle to the bicycle and fixing a chainwheel to the crank arm includes atool-mount member shaped to be held and turned by a tool, wherein thetool-mount member defines an opening for receiving a portion of the axlebody therethrough. An operating member having a plurality of splinesformed on an inside surface thereof is coupled to the tool-mount member.The splines may engage corresponding splines on a mounting adapter forthe crank axle or corresponding splines on a nut used to fix achainwheel to the crank arm. In the latter case, an auxiliary tool maybe provided to ensure stable operation of the tool. The auxiliary toolmay include a grip section disposed in close proximity to a first endthereof, wherein the grip section has a diameter greater than or equalto a diameter of an end of the crank axle. A support section may bedisposed adjacent to the grip section and sized to fit within theopening in the tool-mount member. A second end of the auxiliary tool hasa threaded inner peripheral surface which can engage with the bolt usedto mount the crank arm to the axle. In this case the bolt serves totemporarily fix the main and auxiliary tool to the crank arm so that themain tool can reliably tighten the chainring fixing nut.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a particular embodiment of a crank armaccording to the present invention;

FIG. 2 is a side view of the crank arm shown in FIG. 1;

FIG. 3 is a view taken along line 3—3 in FIG. 1;

FIG. 4 is a rear view of the crank arm shown in FIG. 1;

FIGS. 5A-5C are cross sectional views taken along lines 5A—5A, 5B—5B and5C—5C, respectively, in FIG. 1;

FIG. 6 is an oblique view of a particular embodiment of a crank axleaccording to the present invention for use with the crank arm shown inFIG. 1;

FIG. 7 is a cross sectional view of right and left side crank armsattached to the crank axle shown in FIG. 6;

FIG. 8 is a cross sectional view illustrating a particular embodiment ofa crank arm assembly according to the present invention including a boltused to fix the crank arms to the crank axle;

FIG. 9 is a partial cross sectional view of alternative embodiment ofthe bolt shown in FIG. 8;

FIG. 10 is an oblique view of the bolt shown in FIG. 9;

FIG. 11 is a partial cross sectional view of a crank axle according tothe present invention mounted to a bicycle frame;

FIG. 12 is a cross sectional view illustrating a main tool and anauxiliary tool according to the present invention used to fixchainwheels to the crank arm shown in FIG. 3;

FIG. 13 is an oblique view of the main tool shown in FIG. 12;

FIG. 14 is an end view of the tool shown in FIG. 13; and

FIG. 15 is a partial cross sectional view of the auxiliary tool shown inFIG. 12.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 is a front view of a particular embodiment of a crank arm 1according to the present invention. In this embodiment, the crank arm 1is fabricated from an aluminum alloy. As shown in FIG. 1, the crank armend on the pedal mounting side 4 is narrow, and the crank arm end on thecrank axle mounting side 2 is wide. Varying the width of the crank arm 1depending on the position in this way ensures that the stress bearing oncrank arm is essentially constant at any point. A chamfered section 11is formed at the front (outer) side 3A of the crank arm 1. The size ofthe chamfer is greater in proximity to the crank arm end on the crankaxle mounting side 2 and becomes continuously smaller towards the crankarm end on the pedal mounting side 4. The front surface of the crank arm1 can be inscribed with the manufacturer's logo, a trademark indicatingthe product name, or the like using laser marking or other means.

A crank axle mounting hole 21 (defining a mounting hole axis X) formounting the crank arm 1 onto the crank axle 5 is formed on the crankaxle mounting side 2 of the crank arm 1. The crank arm 1 is mounted ontothe crank axle 5 using this crank axle mounting hole 21 as illustratedin FIG. 6. A pedal mounting hole 41 is formed at the crank arm end onthe pedal mounting side 4 of the crank arm 1. The pedal (not shown) ismounted in this pedal mounting hole 41.

FIG. 2 is a side view of the crank arm shown in FIG. 1, and FIG. 3 is alongitudinal cross sectional view of the crank arm shown in FIG. 1. Asshown in those figures, a groove 31 that extends in the lengthwisedirection is formed on the back (inner) side 3B of the central section 3of the crank arm. The shape of the groove 31 is such that in proximityto the two ends of the groove 31, the depth of the groove becomesprogressively smaller towards each end, with the groove depth reaching aminimum at each end. Thus, the bottom surface of the groove may betermed a “ship hull shape”. The bottom surface of the groove 31 in itscentral section is essentially parallel to the front surface of thecentral section 3 of the crank arm.

A stepped section 32 is formed at the open side of the groove 31. A capmember 33 is inserted into this stepped section 32, and the cap member33 is affixed at its perimeter to the body of the crank arm 1 by inertgas welding using argon gas. The front surface of the body of the crankarm 1 and the surface of the cap member 33 are flush. The pad producedin welding should be removed by machining or other means in order tomake the surface flush.

A crank axle mounting hole 21 is formed on the crank axle mounting side2. The inside wall of the crank axle mounting hole 21 has a flange 22that projects inward, and female splines 23 which are contiguous withthe back surface of this flange 22. The section of the crank axlemounting hole 21 located closer to the back surface than the splines 23constitutes the centering member 24 that defines a circular opening (O).The centering member 24 can be straight, with an inside diameter thatdoes not vary in the axial direction, but more commonly has a taperedshape whose inside diameter flares out towards the back. The taper angleis 2 to 3°. As indicated more specifically in FIG. 8, each of theplurality of female splines (23) comprises an inner axial side surface(231) facing in a direction of an inner side (111) of the crank axlemounting section (2), an outer axial side surface (232) facing in adirection of an outer side (112) of the crank axle mounting section (2),and a radially innermost surface (233) extending in the direction of themounting hole axis. A distance (D1) between the inner axial side surface(231) and the inner side (111) of the crank axle mounting section (2) isgreater than a distance (D2) between the outer axial side surface (232)and the outer side (112) of the crank axle mounting section (2). Also, adistance (D5) between the inner axial side surface (231) and the outeraxial side surface (232) is smaller than a distance (D1) between theinner axial side surface (231) and the inner side (111) of the crankaxle mounting section (2). As shown in FIG. 4, each of the plurality offemale splines further comprises a radially straight firstcircumferential side surface (236) and an opposite radially straightsecond circumferential side surface (238), each of which are positionedessentially perpendicular to the rotational direction of the crank arm.Radially innermost surface (233) extends from first circumferential sidesurface (236) to second circumferential side surface (237). A splinegroove surface (238) is disposed adjacent the first circumferential sidesurface (236) and the second circumferential side surface (237) andradially outwardly therefrom, wherein a diameter (D3) of the crank axlemounting hole (21) at the centering structure (24) is not greater than adiameter (D4) of the crank axle mounting hole (21) at the spline groovesurface (238) as shown in FIG. 3. The centering structure (24) comprisesa circular centering surface (241) that originates from a radiallyoutermost end (235) of the inner axial side surface (231), parallel tothe radially innermost surface (233), and essentially perpendicular tothe inner axial side surface (231). Male splines 25 for locking thechainring 6 in place are formed on the outer periphery of the basalportion of the protruding section located on the back surface of thecrank arm end of the crank axle mounting side 2. A thread 26 foraffixing the chainring 6 is formed on the outer periphery of theprotruding section on the back surface.

FIG. 4 shows the crank arm 1 viewed from the back. As shown in FIG. 4,the splines 23 and 25 each comprise eight teeth. Too few teeth resultsin inadequate linkage strength on the part of the rotary linkage member.Providing too many teeth requires complex machining and entails highercosts, and increases the probability of errors in distribution andpositioning in the direction of rotation.

FIGS. 5A-5C are cross sectional views taken along lines 5A—5A, 5B—B and5C—5C, respectively, in FIG. 1. As shown in those figures, the groove 31constitutes the majority of the cross section interior in the centralsection 3 of the crank arm in order to decrease the weight.

FIG. 6 shows the exterior of the crank axle 5. A centering member 52 isformed at each end of the crank axle 5. The centering members 52 can bestraight, with a diameter that does not vary, but they usually have atapered shape which narrows at the ends. The taper angle is 2 to 3°.Male splines 51 are provided further towards the end from the centeringmembers 52. The splines 51 have an eight-tooth design. The peripheralsurfaces of the splines form a continuous surface with the peripheralsurfaces of the centering members 52. The shape of each spline 51 isbasically square, with the planes of the sides being essentiallyparallel. The linking force in the direction of rotation is borne byplanes which are essentially orthogonal to the direction of force, sochatter is largely eliminated.

The end surfaces of the crank axle 5 are provided with internal threads53 for affixing the crank arms 1. The crank axle 5 is a hollow pipewhose wall is thinner in the central section than at the two ends, asshown in FIG. 7. In this embodiment, an aluminum alloy is used as thematerial for the crank axle 5. This, together with the fact that theaxle is hollow, contributes to reduced weight.

FIG. 7 depicts the crank axle 5, the crank arms 1,100, and the chainring6 in assembled form. The chainring 6 is mounted onto the crank arm 1 onone side, locked in place by the splines 25, and fixed with a nut 61.The crank arm 1 to which the chainring 6 has been affixed is mountedonto one end of the crank axle 5 and affixed by a bolt 54. The crank arm100 without a chainring attached is affixed to the other end of thecrank axle 5. The centering members 24 of the crank arms 1,100 fittightly with the centering members 52 of the crank axle 5 so that thecomponents are centered with high precision. The female splines 23 ofthe crank arms 1,100 mesh with the male splines 51 of the crank axle 5to afford a linkage having high strength in the direction of rotation.

The centering members 52 of the aforementioned crank axle 5 and thecentering members 24 of the crank arms 1,100 have tapered junctions, sothe two tapered surfaces fit together tightly. It is therefore necessaryto use a removal tool in order to remove the crank arm 1,100 from thecrank axle 5. However, the alternative embodiment depicted in FIG. 8 isdesigned so that a bolt 54 that fastens the crank axle 5 and the crankarm 1 (or 100) can be used to release the components.

As shown in FIG. 12, an integrated flange 55 is formed on the head ofthe bolt 54. When fastening the crank axle 5 and the crank arm 1together, a washer 56 is inserted and the bolt 54 is tightened. A stopring 58 is inserted into and retained by a groove 57 located on theinside surface of the crank axle mounting hole 21. To remove the crankarm 1 from the crank axle, a hexagonal Allen key is inserted into thehexagonal hole 59 in the crank arm fixing bolt 54 and turned in thereverse direction. The flange 55 of the bolt 54 then pushes the sidesurface of the stop ring 58. The flange 55 thus forces the crank arm 1in the axial direction, so the tight-fitting tapered junctions of thecentering members 52 of the crank axle 5 and the centering members 24 ofthe crank arms 1 can be easily released. Thus, the need for a specialtool such as an extractor is obviated.

FIG. 9 is a partial cross sectional view of alternative embodiment ofthe bolt shown in FIG. 8, and FIG. 10 is an oblique view of the boltshown in FIG. 9. In the bolt 54 depicted in FIG. 8, the diameter of thethread section and the diameter of the head are about the same, but inthe bolt 54 depicted in FIG. 9 and 10, the diameter of the threadsection 542 is greater than the diameter of the head 541, and thediameter of the flange 55 is greater than the diameter of the threadsection 542. The provision of a thread section 542 with a large diameterensures sufficient strength on the part of the thread section even whenthe component is fabricated from a light alloy such as an aluminumalloy, and affords adequate tightening force on the part of the bolt 54.

A hexagonal hole 59 formed in the center of the end of the head 541extends in the direction of the bolt axis. The hexagonal hole 59 extendsalmost to the distal end of the bolt 54, such that its bottom surface islocated in proximity to the end surface. A longer hexagonal hole 59provides a greater area of contact between the tightening tool (an Allenkey) and the hexagonal hole 59 so that adequate tightening can beachieved even with a light alloy bolt. The length of the hexagonal hole59 can be chosen in accordance with the tightening force required, butit is necessary for the length to be at least such that the bottomsurface of the hexagonal hole 59 extends to the inside surface of thethread section 542. The diameter of the thread section 542 is ratherthick, so the hexagonal hole 59 can extend into its interior; it hasadequate strength even when the hexagonal hole 59 is extended into itsinterior. When fabricated from an aluminum alloy, the bolt 54 has bothlight weight and adequate strength.

FIG. 11 depicts the crank axle 5 which pertains to the present inventioninstalled in the bicycle frame 10. The crank axle 5 pertaining to thisembodiment is provided with a retainer member 50 which protrudes fromthe center of its outside surface for retaining the bearings. To installthe crank axle 5 in the frame 10, a left mounting adapter 101 and aright mounting adapter 102 are threaded between the frame 10 and thecrank axle 5 so that the bearings are retained by the retainer members50. The outsides of the ends of the left mounting adapter 101 and aright mounting adapter 102 are provided with male splines. These malesplines mesh with the female splines of the assembly tool 7 so that eachadapter can be screwed in.

The design of an assembly tool 7 is shown in FIG. 13. The end facing theviewer is provided with an essentially regular hexagonal wrench mountmember 71. A wrench or other tool is fitted over the wrench mount member71, and the entire assembly tool 7 is rotated around its axis. A middlesection 72 is located adjacent to the wrench mount member 71. Theperipheral surface of the middle section 72 is cylindrical, and is cutaway at opposing faces to produce flat surfaces. Tools such as wrenchesof different sizes can be fitted over the flat sections of the middlesection 72. The other end of the assembly tool 7 has a large-diametersection 73.

FIG. 14 shows the assembly tool 7 viewed from the large-diameter section73 side. The inside of the large-diameter section 73 takes the form ofan open cavity. As shown in FIG. 14, female splines 74 are located inthe interior. These female splines 74 mesh with the male splines of theleft mounting adapter 101 and the right mounting adapter 102. Athrough-hole 75 which extends in the axial direction is located in thecenter of the assembly tool 7. The diameter of the through-hole 75 issuch that the end of the crank axle 5 will pass through it. When theleft and right mounting adapters are screwed in, the ends of the crankaxle 5 are passed through the through-holes 75 so that the crank axle 5serves as a guide for rotation of the assembly tool 7, thus facilitatingthe assembly operation.

FIG. 12 shows the crank arm 1 with the chainring 6 attached. Asillustrated in FIG. 12, the chainring 6 is mounted onto the crank armend on the crank axle mounting side 2 of the crank arm 1 and fixed by anut 61. The peripheral surface of the nut 61 is provided with malesplines having the same pitch as the male splines of the left mountingadapter 101 and the right mounting adapter 102, as shown in FIG. 11.Thus, the nut 61 can be tightened using the same assembly tool 7 as thatshown in FIG. 11.

An auxiliary assembly tool 8 is also used in the tightening operation inorder to prevent the assembly tool 7 from coming off during tightening.The design of the auxiliary 10 assembly tool 8 is shown in FIG. 15. Oneend of the approximately cylindrical auxiliary assembly tool 8 isprovided with a grip section 81 whose outside surface has been knurledto prevent slippage. The diameter of the grip section 81 is greater thanthe diameter of the through-hole 75 of the assembly tool 7, and it isdesigned to retained the assembly tool 7. A inside surface supportsection 82 is formed next to the grip section 81. The diameter of theinside surface support section 82 is about the same as the diameter ofthe end of the crank axle 5. Thus, the inside surface of thethrough-hole 75 in the assembly tool 7 can be rotatably supported by theinside surface support section 82. The other end of the auxiliaryassembly tool 8 is provided with a threaded section 83 into which thebolt 54 can be screwed from the end. The length of the auxiliaryassembly tool 8 has been designed such that when the auxiliary assemblytool 8 has been fixed to the crank arm 1 by the bolt 54, there is aslight gap (about 0.5 mm) between the grip section 81 and the assemblytool 7. Thus, the assembly tool 7 is rotatably supported by theauxiliary assembly tool 8 in such a way that it will not come off fromthe nut 61. Thus, a wrench or other tool can be fitted onto the wrenchmount member 71 and the nut 61 can be tightened to fix the chainring 6to the crank arm 1 while the assembly tool 7 is supported by theauxiliary assembly tool 8. Since the assembly tool 7 is prevented fromcoming off from the nut 61, the operation may be conducted easily andefficiently.

While the above is a description of various embodiments of the presentinvention, further modifications may be employed without departing fromthe spirit and scope of the present invention. For example, in theembodiment shown in FIG. 8, a stop ring 58 was provided in crank arm 1.However, an integrated member which corresponds in function to the stopring 58 may be formed on the crank arm 1 instead of the stop ring 58, orsome other component may be welded on. Thus, the scope of the inventionshould not be limited by the specific structures disclosed. Instead, thetrue scope of the invention should be determined by the followingclaims. Of course, although labeling symbols are used in the claims inorder to facilitate reference to the figures, the present invention isnot intended to be limited to the constructions in the appended figuresby such labeling.

What is claimed is:
 1. A bicycle crank arm comprising: a crank arm bodyhaving: a central section (3); a one-piece crank axle mounting section(2) disposed on one end of the central section (3), the crank axlemounting section (2) having an inner peripheral surface (241) that iscontinuous along its entire circumference and defines a crank axlemounting hole (21) having a mounting hole axis (X), including: anunsplined centering structure (24) for centering the crank axle mountingsection (2) to an end of a crank axle (5); and a rotary linkage member(23) including a plurality of female splines (23) disposed in a circlefor nonrotatably fitting the crank axle mounting section (2) to the endof the crank axle (5); wherein the centering structure (24) is disposedbetween the rotary linkage member (23) and an inner side (3B) of thecrank arm body.
 2. The crank arm according to claim 1 wherein thecentering structure (24) is disposed adjacent to the rotary linkagemember (23).
 3. The crank arm according to claim 2 wherein the centeringstructure (24) defines a circular opening (O).
 4. The crank armaccording to claim 1 wherein the plurality of female splines (23) areaxially straight and have a diameter that does not vary in an axialdirection.
 5. The crank arm according to claim 1 wherein the pluralityof female splines (23) and the centering structure (24) are tapered tohave a progressively larger diameter toward the inner side (3B) of thecrank arm.
 6. The crank arm according to claim 1 wherein the rotarylinkage member (23) comprises only eight female splines (23).
 7. Thecrank arm according to claim 1 further comprising a pedal mountingsection (4) disposed on an opposite end of the central section (3). 8.The crank arm according to claim 7 wherein the pedal mounting section(4) includes a pedal mounting hole (41).
 9. The crank arm according toclaim 1 wherein the centering structure (24) is disposed adjacent to therotary linkage member (23), and wherein the centering structure (24)defines a circular opening (O).
 10. (Amended) The crank arm according toclaim 1 further comprising a plurality of male splines (25) disposed onan outer peripheral surface (225) of the crank axle mounting section(2).
 11. The crank arm according to claim 10 wherein the crank axlemounting section (2) has a threaded outer peripheral surface (26) inclose proximity to the plurality of male splines (25).
 12. The crank armaccording to claim 1 wherein each of the plurality of female splines(23) comprises an inner axial side surface (231) facing in a directionof an inner side (111) of the crank axle mounting section (2), an outeraxial side surface (232) facing in a direction of an outer side (112) ofthe crank axle mounting section (2), and a radially innermost surface(233) extending in the direction of the mounting hole axis (X).
 13. Thecrank arm according to claim 12 wherein, for at least one of theplurality of female splines (23), a distance (D1) between the inneraxial side surface (231) and the inner side (111) of the crank axlemounting section (2) is greater than a distance (D2) between the outeraxial side surface (232) and the outer side (112) of the crank axlemounting section (2).
 14. The crank arm according to claim 12 wherein,for at least one of the plurality of female splines (23), the centeringstructure (24) comprises a centering surface (241) that originates froma radially outermost end (235) of the inner axial side surface (231) andparallel to the radially innermost surface (233).
 15. The crank armaccording to claim 14 wherein, for the at least one of the plurality offemale splines (23), the centering surface (241) is essentiallyperpendicular to the inner axial side surface (231).
 16. The crank armaccording to claim 12 wherein, for at least one of the plurality offemale splines (23), a distance (D5) between the inner axial sidesurface (231) and the outer axial side surface (232) is smaller than adistance (D1) between the inner axial side surface (231) and the innerside (111) of the crank axle mounting section (2).
 17. The crank armaccording to claim 1 wherein each of the plurality of female splines(23) has a circumferential side surface (236) essentially perpendicularto a rotation direction of the crank arm.
 18. The crank arm according toclaim 1 wherein the plurality of female splines (23) are tapered to havea progressively larger diameter toward the inner side (3B) of the crankarm.
 19. The crank arm according to claim 1 wherein the centeringstructure (24) is tapered to have a progressively larger diameter towardthe inner side (3B) of the crank arm.
 20. A bicycle crank armcomprising: a crank arm body having: a central section (3); a crank axlemounting section (2) disposed on one end of the central section (3), thecrank axle mounting section (2) defining a crank axle mounting hole (21)with a mounting hole axis (X) including: a centering structure (24) forcentering the crank axle mounting section (2) to an end of a crank axle(5); and a rotary linkage member (23) including a plurality of femalesplines (23) for nonrotatably fitting the crank axle mounting section(2) to the end of the crank axle (5); and a plurality of male splines(25) disposed on an outer peripheral surface (225) of the crank axlemounting section (2).
 21. The crank arm according to claim 20 whereinthe crank axle mounting section (2) has a threaded outer peripheralsurface (26) in close proximity to the plurality of male splines (25).22. The crank arm according to claim 20 wherein the plurality of femalesplines (23) are axially straight and have a diameter that does not varyin an axial direction.
 23. The crank arm according to claim 20 whereinthe plurality of female splines (23) and the centering structure (24)are tapered to become progressively larger toward an inner side (3B) ofthe crank arm.
 24. The crank arm according to claim 20 wherein therotary linkage member (23) comprises only eight female splines (23). 25.The crank arm according to claim 20 further comprising a pedal mountingsection (4) disposed on an opposite end of the central section (3). 26.The crank arm according to claim 25 wherein the pedal mounting section(4) includes a pedal mounting hole (41).
 27. The crank arm according toclaim 20 wherein the centering structure (24) is disposed adjacent tothe rotary linkage member (23), and wherein the centering structure (24)defines a circular opening (O).
 28. The crank arm according to claim 20wherein each of the plurality of female splines (23) comprises an inneraxial side surface (231) facing in a direction of an inner side (111) ofthe crank axle mounting section (2), an outer axial side surface (232)facing in a direction of an outer side (112) of the crank axle mountingsection (2), and a radially innermost surface (233) extending in thedirection of the mounting hole axis (X).
 29. The crank arm according toclaim 28 wherein, for at least one of the plurality of female splines(23), a distance (D1) between the inner axial side surface (231) and theinner side (111) of the crank axle mounting section (2) is greater thana distance (D2) between the outer axial side (232) surface and the outerside (112) of the crank axle mounting section (2).
 30. The crank armaccording to claim 28 wherein, for at least one of the plurality offemale splines (23), the centering structure (24) comprises a centeringsurface (241) that originates from a radially outermost end (235) of theinner axial side surface (231) and parallel to the radially innermostsurface (233).
 31. The crank arm according to claim 30 wherein, for theat least one of the plurality of female splines (23), the centeringsurface (241) is essentially perpendicular to the inner axial sidesurface (231).
 32. The crank arm according to claim 28 wherein, for atleast one of the plurality of female splines (23), a distance (D5)between the inner axial side surface (231) and the outer axial sidesurface (232) is smaller than a distance (D1) between the inner axialside surface (231) and the inner side (111) of the crank axle mountingsection (2).
 33. The crank arm according to claim 20 wherein each of theplurality of female splines (23) has a circumferential side surface(236) essentially perpendicular to a rotation direction of the crankarm.
 34. A bicycle crank arm comprising: a crank arm body having: acentral section (3); a crank axle mounting section (2) disposed on oneend of the central section (3), the crank axle mounting section (2)defining a crank axle mounting hole (21) with a mounting hole axis (X)including: a centering structure (24) for centering the crank axlemounting section (2) to an end of a crank axle (5); a rotary linkagemember (23) including a plurality of female splines (23) fornonrotatably fitting the crank axle mounting section (2) to the end ofthe crank axle (5); wherein each of the plurality of female splines (23)comprises an inner axial side surface (231) facing in a direction of aninner side (111) of the crank axle mounting section (2), an outer axialside surface (232) facing in a direction of an outer side (112) of thecrank axle mounting section (2), and a radially innermost surface (233)extending in the direction of the mounting hole axis (X); wherein, forat least one of the plurality of female splines (23), a distance (D1)between the inner axial side surface (23 1) and the inner side (111) ofthe crank axle mounting section (2) is greater than a distance (D2)between the outer axial side surface (232) and the outer side (112) ofthe crank axle mounting section (2); and wherein the centering structure(24) extends substantially from the inner axial side surface (231) tothe inner side (111) of the crank axle mounting section (2).
 35. Thecrank arm according to claim 34 wherein, for each of the plurality offemale splines (23), a distance (D1) between the inner axial sidesurface (231) and the inner side (111) of the crank axle mountingsection (2) is greater than a distance (D2) between the outer axial sidesurface (232) and the outer side (112) of the crank axle mountingsection (2).
 36. The crank arm according to claim 34 wherein theplurality of female splines (23) are axially straight and have adiameter that does not vary in an axial direction.
 37. The crank armaccording to claim 34 wherein the plurality of female splines (23) andthe centering structure (24) are tapered to become progressively largertoward an inner side (3B) of the crank arm.
 38. The crank arm accordingto claim 34 wherein the rotary linkage member (23) comprises only eightfemale splines (23).
 39. The crank arm according to claim 34 furthercomprising a pedal mounting section (4) disposed on an opposite end ofthe central section (3).
 40. The crank arm according to claim 39 whereinthe pedal mounting section (4) includes a pedal mounting hole (41). 41.The crank arm according to claim 34 wherein the centering structure (24)is disposed adjacent to the rotary linkage member (23), and wherein thecentering structure (24) defines a circular opening (0).
 42. The crankarm according to claim 34 further comprising a plurality of male splines(25) disposed on an outer peripheral surface (225) of the crank axlemounting section (2), wherein the crank axle mounting section (2) has athreaded outer peripheral surface (26) in close proximity to theplurality of male splines (25).
 43. The crank arm according to claim 34wherein, for at least one of the plurality of female splines (23), thecentering structure (24) comprises a centering surface (241) thatoriginates from a radially outermost end (235) of the inner axial sidesurface (231) and parallel to the radially innermost surface (233). 44.The crank arm according to claim 43 wherein, for the at least one of theplurality of female splines (23), the centering surface (241) isessentially perpendicular to the inner axial side surface (231).
 45. Thecrank arm according to claim 34 wherein each of the plurality femalesplines (23) has a circumferential side surface (235) essentiallyperpendicular to a rotation direction of the crank arm.
 46. The crankarm according to claim 34 wherein the outer axial side surface (232) isspaced inward from the outer side (112) of the crank axle mountingsection (2).
 47. The crank arm according to claim 46 further comprisingan annular flange (22) disposed at the outer axial side (232) of each ofthe plurality of female splines (23).
 48. The crank arm according toclaim 34 wherein, for the at least one of the plurality of femalesplines (23), a distance (D5) between the inner axial side surface (231)and the outer axial side surface (232) is smaller than a distance (D1)between the inner axial side surface (231) and the inner side (111) ofthe crank axle mounting section (2).
 49. The crank arm according toclaim 34 wherein the centering structure (24) has an inner peripheralsurface (241) that is continuous along its entire circumference.
 50. Abicycle crank arm comprising: a crank arm body having: a central section(3); a one-piece crank axle mounting section (2) disposed on one end ofthe central section (3), the crank axle mounting section (2) having aninner peripheral surface defining a completely bounded crank axlemounting hole (21) including: a centering structure (24) for centeringthe crank axle mounting section (2) to an end of a crank axle (5); arotary linkage member (23) including a plurality of female splines (23)for nonrotatably fitting the crank axle mounting section (2) to the endof the crank axle (5); and wherein each of the plurality of femalesplines (23) comprises a radially inwardly extending firstcircumferential side surface (236); a radially inwardly extending secondcircumferential side surface (237) opposite the first circumferentialside surface (236), a radially innermost surface (233) extending betweenthe first circumferential side surface (236) and the secondcircumferential side surface (237), and a spline groove surface (238)adjacent the first circumferential side surface (236) and the secondcircumferential side surface (237) and radially outwardly therefrom. 51.The crank arm according to claim 50 wherein the centering structure (24)comprises an unsplined surface (241).
 52. The crank arm according toclaim 51 wherein the centering structure (24) comprises a circularsurface (241).
 53. The crank arm according to claim 50 wherein adiameter (D3) of the crank axle mounting hole (21) at the centeringstructure (24) is not greater than a diameter (D4) of the crank axlemounting hole (21) at the spline groove surface (238).
 54. The crank armaccording to claim 53 wherein the diameter (D3) of the crank axlemounting hole (21) at the centering structure (24) is substantiallyequal to the diameter (D4) of the crank axle mounting hole (21) at thespline groove surface (238).
 55. The crank arm according to claim 50wherein the first circumferential side surface (236) is radiallystraight.
 56. The crank arm according to claim 55 wherein the secondcircumferential side surface (237) is radially straight.
 57. The crankarm according to claim 50 wherein each of the plurality of femalesplines (23) comprises an inner axial side surface (231) facing in adirection of an inner side (111) of the crank axle mounting section (2)and an outer axial side surface (232) facing in a direction of an outerside (112) of the crank axle mounting section (2).
 58. The crank armaccording to claim 57 wherein, for at least one of the plurality offemale splines (23), a distance (D1) between the inner axial sidesurface (231) and an inner side (111) of the crank axle mounting section(2) is greater than a distance (D2) between the outer axial side surface(232) and an outer side (112) of the crank axle mounting section (2).59. The crank arm according to claim 58 wherein the outer axial sidesurface (232) is spaced inward from the outer side (112) of the crankaxle mounting section (2).
 60. The crank arm according to claim 59further comprising an annular flange (22) disposed at the outer axialside (232) of each of the plurality of female splines (23).
 61. Thecrank arm according to claim 58 wherein, for the at least one of theplurality of female splines (23), a distance (D5) between the inneraxial side surface (231) and the outer axial side surface (232) issmaller than a distance (D1) between the inner axial side surface (231)and the inner side (111) of the crank axle mounting section (2).
 62. Thecrank arm according to claim 50 wherein, for at least one of theplurality of female splines (23), the centering structure (24) comprisesa centering surface (241) that originates from a radially outermost end(235) of the inner axial side surface (231) and parallel to the radiallyinnermost surface (233).
 63. The crank arm according to claim 62wherein, for the at least one of the plurality of female splines (23),the centering surface (241) is essentially perpendicular to the inneraxial side surface (231).
 64. The crank arm according to claim 50wherein the centering structure (24) is disposed between the rotarylinkage member (23) and an inner side (111) of the crank axle mountingsection (2).
 65. The crank arm according to claim 50 wherein the rotarylinkage member (23) comprises only eight female splines (23).
 66. Abicycle crank arm comprising: a crank arm body having: a central section(3); a one-piece crank axle mounting section (2) disposed on one end ofthe central section (3), the crank axle mounting section (2) having aninner peripheral surface defining a completely bounded crank axlemounting hole (21) including: a centering structure (24) comprising acircular unsplined surface (241) for centering the crank axle mountingsection (2) to an end of a crank axle (5); a rotary linkage member (23)including a plurality of female splines (23) for nonrotatably fittingthe crank axle mounting section (2) to the end of the crank axle (5);wherein each of the plurality of female splines (23) comprises aradially inwardly extending first circumferential side surface (236); aradially inwardly extending second circumferential side surface (237)opposite the first circumferential side surface (236), a radiallyinnermost surface (233) extending between the first circumferential sidesurface (236) and the second circumferential side surface (237), and aspline groove surface (238) adjacent the first circumferential sidesurface (236) and the second circumferential side surface (237) andradially outwardly therefrom; and wherein a diameter (D3) of the crankaxle mounting hole (21) at the centering structure (24) is substantiallyequal to a diameter (D4) of the crank axle mounting hole (21) at thespline groove surface (238).
 67. The crank arm according to claim 66wherein the centering structure (24) is disposed between the rotarylinkage member (23) and an inner side (111) of the crank axle mountingsection (2).
 68. The crank arm according to claim 66 wherein the rotarylinkage member (23) comprises only eight female splines (23).
 69. Thecrank arm according to claim 66 wherein each of the plurality of femalesplines (23) comprises an inner axial side surface (231) facing in adirection of an inner side (111) of the crank axle mounting section (2)and an outer axial side surface (232) facing in a direction of an outerside (112) of the crank axle mounting section (2).
 70. The crank armaccording to claim 69 wherein, for at least one of the plurality offemale splines (23), a distance (D1) between the inner axial sidesurface (231) and an inner side (111) of the crank axle mounting section(2) is greater than a distance (D2) between the outer axial side surface(232) and an outer side (112) of the crank axle mounting section (2).71. The crank arm according to claim 69 wherein, for at least one of theplurality of female splines (23), a distance (D5) between the inneraxial side surface (231) and the outer axial side surface (232) issmaller than a distance (D1) between the inner axial side surface (231)and the inner side (111) of the crank axle mounting section (2).